In such an apparatus the source of pressurized fluid (hydraulic power) is quite expensive. For this reason, when a vehicle has power assisted steering, as for example described in the specification of French published patent application FR 2 448 472A which corresponds to U.S. Pat. No. 4,414,883, it is desirable to use the hydraulic power source of the pressurized fluid in the power steering system for also supplying fluid to the servo means in another installation on the vehicle, such as a servo assisted clutch. However, this gives rise to a problem due to the fact that the supply of the servo means must not be such as to disturb the operation of the power assisted steering in any way.